This application claims the benefit of Korea Application No. 2002-46167, filed Aug. 5, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a magnetron for microwave ovens, and more particularly, to a vane of a magnetron for microwave ovens.
2. Description of the Related Art
Generally, a magnetron is constructed to have an anode and a cathode such that thermions are discharged from the cathode and spirally moved to the anode by electromagnetic force. A spinning electron pole is generated around the cathode by the thermions and current is induced in an oscillation circuit of the anode, so that oscillation is continuously stimulated. An oscillation frequency of the magnetron is generally determined by the oscillation circuit, and has high efficiency and high output power. The magnetron is widely used in home appliances, such as microwave ovens, as well as in industrial applications, such as high-frequency heating apparatuses, particle accelerators and radar systems.
The general construction and operation of the above-described magnetron are briefly described with reference to FIGS. 1 through 3.
As shown in FIG. 1, the magnetron generally includes a positive polar cylinder 101 made of an oxygen free copper pipe or the like, a plurality of vanes 102 disposed in the positive polar cylinder 101 to constitute a positive polar section along with the positive polar cylinder 101 and radially arranged at regular intervals to form a cavity resonator, and an antenna 103 connected to one of the vanes 102 to induce harmonics to an outside. The magnetron also includes a large-diameter strip ring 104 and a small-diameter strip ring 105 disposed on upper and lower portions of the vanes 102, respectively, to alternately and electrically connect the vanes 102 so that the vanes 102 alternately have the same electric potential as shown in FIG. 2.
Rectangular depressions 202 are formed in the vanes 102, respectively, to allow the strip rings 104 and 105 to alternately and electrically connect the vanes 102, and cause each opposite pair of the vanes 102 to be disposed in an inverted manner. According to the above-described construction, each of the pair of opposite vanes 102 and the positive polar cylinder 101 constitute a certain LC resonant circuit. Additionally, a filament 106 in a form of a coil spring is disposed in an axial center portion of the positive polar cylinder 101, and an activating space 107 is provided between radially inside ends of the vanes 102 and the filament 106. An upper shield 108 and a lower shield 109 are attached to a top and bottom of the filament 106, respectively. A center lead 110 is welded to a bottom of the upper shield 108 while being passed through a through hole of the lower shield 109 and the filament 106. A side lead 111 is welded to a bottom of the lower shield 109. The center lead 110 and the side lead 111 are connected to terminals of an external power source (not shown), and therefore, forms a closed circuit in the magnetron.
An upper permanent magnet 112 and a lower permanent magnet 113 are provided to apply a magnetic field to the activating space 107 with opposite magnetic poles of the upper and lower permanent magnets 112 and 113 facing each other. An upper pole piece 117 and a lower pole piece 118 are provided to induce rotating magnetic flux generated by the permanent magnets 112 and 113 into the activating space 107. The above-described elements are enclosed in an upper yoke 114 and a lower yoke 115. Cooling fins 116 connect the positive polar cylinder 101 to the lower yoke 115, and radiate heat generated in the positive polar cylinder 101 to the outside through the lower yoke 115.
According to the above-described construction of the magnetron, when power is applied to the filament 106 from the external power source, the filament 106 is heated by operational current supplied to the filament 106, the thermions are emitted from the filament 106, and a group of thermions 301 are produced in the activating space 107 by the emitted thermions as shown in FIG. 3. The group of thermions 301 alternately imparts potential difference to each neighboring pair of the vanes 102 while being in contact with front ends of the vanes 102, being rotated by influence of the magnetic field formed in the activating space 107, and being moved from one state xe2x80x9cixe2x80x9d to another state xe2x80x9cixe2x80x9d. Accordingly, harmonics corresponding to a rotation speed of the thermion group 301 are generated by oscillation of the LC resonant circuit formed by the vanes 102 and the positive polar cylinder 101, and transmitted to the outside through the antenna 103.
Generally, frequency is calculated by an equation
f=1/2xcfx80{square root over (LC)},
where L is an inductance and C is a capacitance. Values of the variables of the above equation are determined by geometrical configurations of circuit elements. Thus, the configurations of the vanes 102 constituting part of the LC resonant circuit are principal factors in determining the frequency of harmonics.
In the magnetron having the above-described construction and operation, noise of a considerably wide band considered as unwanted electromagnetic waves is generated. The noise may induce malfunction in other devices. Thus, a reduction in the noise is an important technical issue that has been researched for a long time. In this regard, the geometrical configuration of the vane, which is one of factors that determine a frequency of electromagnetic waves generated in the magnetron, is an important technical issue relative to the generation of noise.
Conventional vanes constituting parts of the magnetron are constructed as shown in FIG. 4. The shortcomings of the conventional vanes are described with reference to FIG. 4. As shown in FIG. 4, a pair of neighboring vanes is illustrated as being opposite to each other for convenience of explanation.
As shown in FIG. 3, the depressions 202 are formed to allow the strip rings to be disposed therein. In FIG. 4, the depressions 202 are constructed to have rectangular shapes. After the thermions arrive at sections xe2x80x9ca,xe2x80x9d xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d of the front side of a vane 102, the thermions arriving at the section xe2x80x9caxe2x80x9d are moved to the section xe2x80x9ccxe2x80x9d of the front side of another neighboring vane 102 because of the inverted relationship of the pair of neighboring vanes 202. As the thermions arrive at the front side of the vane 102, a potential difference is generated between the pair of neighboring vanes 102 and, current (that is, the flow of thermions) is supplied to the filament 106. The thermions arriving at the sections xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d are moved to the sections xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d of the front side of the neighboring vane 102 along roundabout paths due to a hindrance effect of the depressions 202, thus resulting in delaying the arrival of the thermions at the section xe2x80x9cbxe2x80x9d of the front side of the neighboring vane 102 in comparison with the arrival of the sections xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d.
In FIG. 4, arrows L1, L2 and L3 represent distances along which the thermions travel from one of the vanes 102 to the neighboring vane 102. The thermions at the sections xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d travel along the same distance at the same time. A main frequency of the magnetron is generally determined by the sections xe2x80x9cbxe2x80x9d of the vanes 102. Therefore, the delays in the thermions reaching the sections xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d of the neighboring vane 102 cause noise in all the frequencies of the magnetron.
Accordingly, it is an aspect of the present invention to provide a magnetron for microwave ovens to reduce high frequency noise caused by a difference between velocities of thermions flowing through vanes of the magnetron, thus optimizing frequency of microwaves emitted from the magnetron.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing and other aspects of the present invention are achieved by providing a magnetron for microwave ovens, including a positive polar cylinder, a cathode, and a plurality of vanes to constitute a positive polar section along with the positive polar cylinder. Each of the vanes is provided with a first depression to allow a large-diameter strip ring to be disposed therein, and a second depression to allow a small-diameter strip ring to be disposed therein. Also, the vane is provided at a cathode-side corner of the first depression with a thermion travel passage to allow thermions to smoothly flow without hindrance of the first depression.